1. Field of the Invention
The present invention relates to an automatic focusing apparatus (device), and particularly to an automatic focusing apparatus that detects a focus point using a liquid crystal lens for adjustment of focal length, by extracting plural focus signals corresponding to a focus matching degree from image signals obtained from optical images that are formed through the liquid crystal lens during transient response operation of the liquid crystal lens, and by detecting a maximal value of the focus signal.
2. Description of the Related Art
Conventionally, as a focus mechanism changing the focal length or focus position of an optical system, the system of adjusting the focus by moving a lens has been widely used. However, this system needs a lens drive mechanism and thus has defects that the mechanism becomes complicated and needs comparatively high electric power for the lens-drive motor. Moreover, shock resistance is generally low. Therefore, as a focusing apparatus that does not require the lens drive mechanism, such a system that adjusts the focus by changing the refractive index of a liquid crystal lens has been proposed (for example, see Patent document 1).
The liquid crystal lens used for the focusing apparatus of this method has a configuration of holding a liquid crystal layer between two glass substrates each of which is equipped with a pattern electrode and a common electrode. This pattern electrode has a core electrode and plural ring electrodes, and has a configuration that the core electrode and each of the ring electrodes are connected by a voltage drop resistor. To an extraction electrode that is connected to the core electrode but is insulated from each ring electrode, a variable resistor is connected via a power amplifier, and to an extraction electrode that is connected to a ring electrode (a peripheral electrode), variable resistor is connected via an amplifier. Furthermore, the alternating voltage supplied from the alternating power source connected in parallel with these variable resistors is lowered by the variable resistors.
Thus, the voltage distribution is formed by the voltage signal applied to the extraction electrodes and the voltage drop resistors, and the voltage by the voltage distribution is applied on the liquid crystal layer. And it becomes possible to generate various forms of voltage distribution applied on the liquid crystal layer by adjusting each variable resistor.
As an autofocus (automatic focusing) system for a video camera, a contour detection system is publicly known that extracts information corresponding to blurriness of an image directly from a photograph image signal and carries out hill-climbing control of the lens to minimize the blurriness. Various autofocus devices and the like using this hill-climbing control system have been proposed (for example, see Patent Document 2, 3, 4).
Patent Document 1: JP No. 3047082B
Patent Document 2: JP No. 2742741B
Patent Document 3: JP No. H01-15188B
Patent Document 4: JP No. H02-11068B
However, adjustment of the focus by controlling changes in the refractive index of the liquid crystal lens by the hill-climbing control has not been reported, so far. It is considered that is because the liquid crystal lens takes a long time to detect the focus point by the hill-climbing control. For example, assuming that 50 focus positions are set in advance in from a short- to long-distance view, and assuming that it is necessary to check 25 positions on average until a minimum point is found when the minimum point information corresponding to blurriness is searched in a certain direction, time required to detect the focus point is compared between the method of moving a lens is applied and the method of using a liquid crystal lens is applied.
In the method of moving the lens, such an operation is repeated that the lens is moved to a position corresponding to a certain position and the information corresponding to the blurriness is obtained at this time, then the lens is moved to another position corresponding to a next position and obtains the information corresponding to blurriness. In this case, since the processing time at one position is as short as 67 milliseconds, for example, the time required for detecting the focus point is approximately 1.7 seconds (=67 ms×25 position) on an average.
On the other hand, in the method using the liquid crystal lens, to drive the liquid crystal lens, distribution of the refractive index of the liquid crystal is altered by changing the voltage (driving voltage) applied. Accordingly, such an operation is repeated should be repeated that a driving voltage corresponding to a certain position is applied to the liquid crystal lens to obtain information corresponding to the blurriness at that time, and then a driving voltage corresponding to a next position is applied to the liquid crystal lens to obtain the information corresponding to the blurriness.
However, since the response of the liquid crystal to the driving voltage change is generally delayed, it is necessary to wait after change of the driving voltage until the response of the liquid crystal is stabilized. Therefore, the processing time per one position becomes long, for example 500 milliseconds, and the time to detect a focus point takes approximately 12.5 seconds (500 ms×25 positions) on average, and it is impractical.
Moreover, according to Patent Document 1, the liquid crystal lens has a configuration in which voltages are applied to both ends of the voltage drop resistors, and naturally, there is a case when the voltage applied to one end is lower than that to the other end. For example, when the liquid crystal is expected to act as a convex lens, low voltage is applied to one extraction electrode, and high voltage is applied to another extraction electrode.
In this case, depending on a used liquid crystal material of the liquid crystal layer, the completion time of the transient response of the liquid crystal on the side of lower applied voltage becomes later than the completion time of the transient response of the liquid crystal on the side of higher applied voltage. Thus, for a liquid crystal lens to act as a convex lens, the response time of the liquid crystal on the side with the low applied voltage determines the required time for the crystal to function as a convex lens.
Especially, when trying to pull out the power of the lens to the maximum extent, voltage difference between the core electrode and the peripheral electrode is made the maximum, thus on the side of the liquid crystal layer to which a lower voltage is applied, the lowest possible voltage at which liquid crystal molecules effectively operate is to be used, so that there has been a problem that a long time period is required until the lens becomes to have a suitable refractive index distribution on this side (until the transient response is completed).
Furthermore, when the power of a lens is enlarged as much as possible, the birefringence index of the liquid crystal material or thickness of the liquid crystal layer must be enlarged. However, when such a configuration is adopted, there has been a problem that the response of the liquid crystal becomes slow and a long time period is required until the lens becomes to have a suitable refractive index distribution.